The present invention relates to diplexers and multiplexers for combining the powers of two or more different frequency signals such as different frequency modulated, FM, channels in a single common transmission line or in a single master antenna so that both channels are broadcast from the master antenna. A diplexer incorporating features of the invention may also combine the video and audio power of a TV channel for broadcast by a single master antenna.
Heretofore, diplexers have been provided for combining the power of two different frequency channels and feeding the combined power to a common transmission line and/or a common master antenna. For example, the power of several frequency modulated (FM) radio channels can be broadcast from a single master antenna by combining the powers in a diplexer or in a series of diplexers called a multiplexer, that feeds the master antenna. A similar diplexer has also been used to combine the audio and video power of a TV channel so that both may be broadcast from a single master antenna. Diplexers used in the past are a hybrid circuit including two directional couplers, one for coupling the through frequency channel, ft, and the other for coupling the injected frequency channel, fi, to the master antenna. In order to insure that there is a minimum insertion loss incurred for each channel fed through the diplexer to the antenna, the terminals of the diplexer to which the channels feed power must be electrically isolated from each other. In the past, this has been achieved in several ways.
According to one technique, called the long stub technique, a long line type of stub which is capacitively capped is provided between the two couplers. The length of the long line must be many wave lengths of the carrier frequency and one quarter wave length of the difference frequency between the carriers of the two channels (fi-ft). In a typical case where two FM channels, separated in frequency by 800 KHz, are combined by the diplexer, the stub must be about 370 feet long. This stub, alternately with frequency, puts a very high and very low impedance between the two couplers. The low impedance occurs at the injected frequency, fi, and so that frequency is reflected at the stub, (just as a short circuit reflects), to the output of the diplexer with negligible insertion loss. The high impedance occurs at the through frequency, ft, and so the through frequency passes the stub substantially unaffected and so is conducted through both couplers to the diplexer output with negligible insertion loss. One limitation of this technique is that ft must fall within a preset frequency slot depending on fi-ft and the stub length and, usually, a diplexer of this type cannot be used with more than the two frequencies, one ft and one fi, and it cannot be readily tuned in the field. Furthermore, since the stub is capacitively capped and such capacitance is sensitive to temperature and environmental changes, it is necessary to tune the diplexer frequently to compensate for these changes.
According to another technique, called the direct coupled technique, tuned cavities are provided between the through and injection directional couplers of the hybrid diplexer. These cavities may be either loop coupled or probe coupled and are tuned very tightly to the injected frequency, fi, and are connected as close as possible to the transmission lines between the couplers. Even so, the electrical length of this connection cannot be made zero and so there is always a reactance that has to be tuned out at a fixed frequency. If it is not tuned out, the through frequency signal, ft, (or at least one of the through frequencies) will see a mismatch and be reflected instead of passing through.
The advantage of this direct coupled technique, using tuned cavities, is that any through frequency can be used and the cavities can be tuned readily in the field to reject (reflect) the injected frequency fi. The disadvantages are:
(a) Even if the cavities are easily tuned in the field, the reactance of the cavity connection must be tuned out for each through frequency;
(b) If a through frequency is not tuned out, there will be poor isolation between the through frequency port and the injected frequency port and excessive insertion loss for the through frequency; and
(c) The through frequency will incur different phase shifts in the two transmission lines between the two directional couplers making the aforsaid isolation and insertion loss even worse.
Multiplexers for combining the powers of three or more channels and feeding the combined power to a single master antenna have consisted of a number of diplexers in series, each one in the series, successively adding the power of another FM channel, called the injected channel, to the combined power of the channels added up to that diplexer, called the through channels; and so the powers of all the injected channels are combined at the output of the last diplexer in the series and these are fed to the master antenna. Each diplexer must provide means for insuring a high degree of isolation between the input port thereof carrying the many through channels and the input port thereof carrying the single injected channel.
Where the long stub technique is used, the problems become increasingly more difficult as one progresses along the series, because the number of through channels increases by one at each diplexer, but the frequency band that will pass through the diplexer remains limited to the band between peaks (short circuits) in the frequency characteristic of the long stubb.
Where tuned cavities are used, the band between peaks (short circuits) in the frequency characteristics of the tuned cavity is wide enough, but the reactance of the cavity connection can't be tuned out for all the through frequency channels and so some of those channels experience excessive insertion loss through the diplexer and poor isolation from the inserted frequency port.
One object of the present invention is to provide good electrical isolation and low insertion loss in diplexers and/or multiplexers of the types described above and yet avoid some of the limitations of the prior techniques. Other objects of the present invention and features are described hereinbelow.